This invention relates to a novel method for preparing phosphors that can be used in the viewing screens of television picture tubes.
Color television picture tubes are cathode-ray tubes which employ a luminescent viewing screen that is selectively excited with the cathode rays generated in the tube. The screen usually employs a red-emitting phosphor, a green-emitting phosphor and a blue-emitting phosphor. Various green-emitting phosphors have been employed as the tube has evolved to its present state. Presently, the most common, and brightest, green-emitting phosphor with the desired emission color is copper-and-aluminum-activated zinc-cadmium sulfide (ZnCdS:Cu:Al). Because the use of cadmium appears to be undesirable for environmental safety reasons, attempts have been made to provide suitable cadmium-free green-emitting phosphors. Of these attempts, zinc sulfide activated with copper, aluminum and gold (ZnS:Cu:Al:Au) appears to be the most promising. Our best results were obtained by making the phosphor by a process which includes heating a mixture of metal-ion compounds openly exposed to a gaseous hydrogen sulfide atmosphere at its reaction temperatures. Such processes are difficult to practice on a commercial scale because of the corrosive effects of hydrogen sulfide atmospheres at the required reaction temperatures. Also, the phosphors produced by such processes do not exhibit the desired emission color.
It is known to make ZnCdS:Cu:Al phosphor on a commercial scale by a process which includes heating a mixture of metal-ion compounds, elemental sulfur and elemental carbon in a covered container at its reaction temperatures, thereby avoiding the need for heating a mixture openly exposed to a hydrogen sulfide atmosphere. Due to the large ionic radius of gold ions, it is extremely difficult to incorporate the needed concentrations of gold ions into a zinc-sulfide crystal lattice by this latter process in order to obtain the green emission color that is particularly desired for use in color television picture tubes.